1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance apparatus, as well as a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance tomography is based on the physical phenomenon of nuclear magnetic resonance and has been successfully employed for many years in medicine and biophysics as an imaging method. In this examination method, the examination subject is exposed to a strong, constant magnetic field, referred to as a basic field. As a result, the nuclear spins of the atoms in the subject align, which were previously irregularly oriented. Radio-frequency waves can then excite these xe2x80x9corderedxe2x80x9d nuclear spins to a specific oscillation. In tomography, this oscillation generates the actual measured signal that is picked up with suitable reception coils. In order to be able to acquire signals from the test subject spatially encoded in all three spatial directions, a gradient coil system is provided that usually has three separate gradient coils (x, y and z coils) via which separate, location-dependent magnetic fields can be generated.
The central physical phenomena in a magnetic resonance tomography apparatus are thus the magnetic fields. The magnetic fields are also responsible for the quality of the acquired images and are ultimately responsible for the diagnosis that can be produced therewith. However, there are a number of components (permanent magnets, shim plates, etc.) in a magnetic resonance tomography apparatus having temperature-dependent magnetization. The homogeneity of the basic field is disturbed if temperature fluctuations of these components occur. These parts therefore must be kept stable in temperature since compensation implemented by software of the influence of these components, which usually have a large surface area on the basic field is only possible under certain conditions and unusable image exposures may consequently occur given a field variation.
A dynamic introduction of heat ensues, for example, due to fluctuations in the room temperature or the coolant temperature, due to the time-dependent, ohmic losses in the gradient coils or as a result of eddy current losses in the components themselves. For example, in conventionally planned applications in a closed MR system, the temperature of the components can be permitted to fluctuate by no more than 0.5 K/10 minutes; but injections of power in the range from 200-300 W/m2 frequently occur.
For homogenizing the basic magnetic field, it is known to employ a shim system having a number of shim plates that usually are arranged at the gradient coil system. The shim plates themselves, however, are temperature-sensitive elements, i.e. they heat up during operation. Their magnetic behavior changes due to the heating, which has a disadvantageous influence on the basic field. A positional change of the shim plates due to mechanical deformation also may occur due to the temperature-dependency of the mounting, which also may be caused by the heating of other parts that are mechanically connected to the shim plates.
It is known to keep the temperature of the shim plates constant by means of an active heating of the shim plates, so that the influence caused by temperature variation can be largely compensated.
Even though a notable improvement of the temporal and topical field homogeneity can be achieved by this measure, image sequences show that field changes in the operation of the apparatus nonetheless occur even in the case of relatively successful temperature constancy of the shim plates and the image quality deteriorates.
An object of the present invention is to provide a method that further improves the topical and temporal stability of the basic field homogeneity in the operation of a magnetic resonance apparatus, as well as a magnetic resonance apparatus wherein the topical and temporal stability of the basic field homogeneity is improved.
For solving this problem, in a method for the operation of a magnetic resonance apparatus having a gradient coil system as well as a shim system with a number of shim plates with respective heaters, at least one information signal indicating a change in shape and/or position of the gradient coil system is determined or employed, the regulation of the heaters, and thus the temperature of the shim plates, ensuing dependent thereon.
The regulation of the heaters and thus the shim plate temperature inventively ensues dependent on an information signal that is a criterion for a change in shape and/or position of the gradient coil system. It has been shown that a change in position of the shim plates occurs due to the heating of the gradient coil system that occurs during operation, the change in position leading to a deterioration of the basic field homogeneity. A field drift that has a disadvantageous influence on the acquired image sequences thus begins. By acquiring an indication of this change in shape or position taking it into account in the temperature regulation, this field drift can enter into consideration in the homogenization of the field using the shim plates, since influencing the saturation induction of the shim plates, that in turn allows a compensation of the field drift, is possible dependent on the regulated shim plate temperature. At the same time, of course, temperature stabilization of the shim plates also remains an objective of the heating procedure. It is possible to stabilize the temperature as well as to select the stabilized temperature such that geometry-caused changes of the magnetic fields can be compensated. An adequately precise temporal and topical compensation thus can ensue on the basis of the individual regulation of the individual shim plates.
In an embodiment of the invention the information signal or signals are determined using one or more measurement elements. The measurement element or elements can acquire the magnetic field or fields generated with the gradient coil system, i.e. field changes that arise in situ are directly acquired, for example markers with the MR imaging. Alternatively or additionally, the measurement element or elements can acquire the temperature of the gradient coil system, i.e. suitable temperature sensors are provided at the gradient coil system (which usually includes the coils and a carrier, usually a GFK pipe or a GFK plate into which the coils are cast), the temperature being determined over the entire system. In another alternative expansion or strain sensors that supply the information signals are employed. The strain sensorsxe2x80x94a number thereof preferably being employedxe2x80x94are suitably arranged or aligned such that different types of deformations can be acquired, i.e. expansion, compression and changes in position. Of course, it is also possible to register a number of different information signals and process them in the course of the regulation.
The measurement elements preferably are arranged uniformly distributed over the gradient coil system, so that locally resolved information signals are acquired to enable an exact, temporally and topically resolved regulation.
In a further embodiment of the inventive method the measurement elements are arranged, preferably uniformly distributed at, the outside and the inside of the gradient coil system, so that topically resolved information signals from the outside and the inside can be locally acquired. The coils of the gradient coil system are arranged above one another, at least in sections, i.e. they lie at different distances from the center of the measurement subject. Signal acquisition at both sides of the gradient coil system enables a more exact registration of parameters or information signals, and thus acquisition of possible changes, since the parameters or signals are picked up as close as possible to all coils of the system. This is particularly expedient when the shim plates are likewise provided at the outside and inside, so that when, for example, the z-coil residing at the outside presents difficulties, the shim plate or plates that are adjacent to it at the outside can be suitably regulated.
An alternative to the acquisition of the information signal or signals via suitable measurement elements is to employ at least one operating parameter of the gradient coil system as the information signal. Since the changes in shape and/or position of the gradient coil system essentially arise from the operation thereof, conclusions can be made about the coil or system temperature and, from these, about possible changes in shape or position that enter into the regulation, on the basis of the relevant operating parameter or parameters. An expedient operating parameter is the operating current, i.e. the current sequence of the coils.
An operating parameter that represents a criterion for the cooling of the gradient coil system also can be employed. Despite active auxiliary heating, the gradient coil system continues to be cooled via a cooling device the operating parameters of which, of course, likewise have an influence on the temperature response of the gradient coil system. Any parameter that describes the cooling or the cooling capacity in any form can be employed as operating parameter.
In another embodiment of the inventive method, the information signal is determined by means of an analysis or one or more excitation responses received with the magnetic resonance apparatus. A field inhomogeneity is very quickly expressed within a registered image sequence, so that the field change is acquired quasi in situ by the image analysis, the quality of said field change then being identified and a corresponding information signal being output.
It is also expedient to take the energy consumption of the regulating devices for the shim into consideration in the temperature regulation. The regulating devices likewise consume energy and heat up. Their heat contribution can likewise be employed in the overall regulation.
In a further embodiment of the regulation method, a reference temperature value to which the actual temperature of the shim plates is adjusted be selected dependent on the at least one information signal. For example, a look-up table can be employed for this purpose, the suitable reference value being selected therefrom dependent on the existing information signal or signals. The reference value is forwarded to the regulating device that drives the heater.
In addition to the above-described method, the invention is directed to a magnetic resonance apparatus having a gradient coil system as well as a shim system with a number of shim plates respectively allocated heaters and a regulating device, wherein the operation of the heaters and thus the temperature of the shim plates, can be regulated dependent on at least one information signal indicating a change in shape or position of the gradient coil system.
In an embodiment of the inventive apparatus, each regulating device regulates on the basis of a reference value signal representing a criterion for a reference temperature value, the reference value signal being defined dependent on the information signal, i.e. the regulating device is provided with a reference value signal on the basis of which the regulation of the heaters ensues. Expediently, a control device that is provided with the information signal or signals, and that communicates with the regulating devices, is provided for determining the rated value signal. A look-up table can be stored in the control device, from which the reference temperature value is selected dependent on the information signal or signals and forwarded to the regulating device in the form of the reference value signal.
Because the gradient coils are respectively at different distances with reference to the examination subject, it is expedient when two regulatable heaters are allocated to each shim plate, these being arranged at opposite sides of the shim plate and being separately regulatable via the common regulating device. Since the shim plates are arranged between two gradient coils, particularly in an open system, one of these heaters is thus located relatively close to a gradient coil, so that heating as directly as possible is achieved.
One or more measurement elements that can communicate with the control device can be provided for acquiring the information signal or signals. The measurement elements can be fashioned for acquiring the magnetic fields generated with the gradient coil system, for acquiring the temperature of the gradient coil system or for acquiring expansions or compressions of the gradient coil system as may occur. Preferably, they are arranged uniformly distributed over the gradient coil system, so that locally resolved information signals can be acquired. The number of measurement elements can be arranged at the outside as well as at the inside of the gradient coil system, preferably uniformly distributed, so that locally resolved information signals can be acquired from both sides.
Alternatively or additionally, at least one operating parameter, particularly the operating current of the gradient coil system, can be employed as the information signal. An operating parameter that represents a criterion for the cooling of the gradient coil system can also be processed as information signal.
The inventive magnetic resonance apparatus also can include an analysis unit for determining the information signal by an analysis of one or more images registered with the magnetic resonance apparatus, the analysis unit communicating as warranted with the control device.
Each heater can be fashioned as a foil or film heater having a carrier foil with heat conductors applied thereon, as a plate heater having a carrier plate with heat conductors applied thereon or as a substrate heater with a ceramic, particularly Al2O3 substrate with applied heat conductors. These very flat heaters make it possible to construct the shim plates relatively small despite the integrated heater. The heat conductors themselves are expediently arranged to proceed in bifilar fashion for suppressing magnetic fields that may be generated during their operation, i.e. there are respective forward and return lines, so that magnetic fields caused by a flow of current mutually compensate.